This invention relates generally to the spectral analysis of the composition of materials and is particularly directed to Fourier transform infrared (FTIR) spectroscopy and related methods and apparatus.
Spectroscopic measurements generally involve the measurement of two spectra, a sample and a reference. The reference measurement is required to accurately record the spectroscopic response of the spectrometer and perhaps the response of a reference cell or control sample. In simplest form, a "single beam" approach is used. The reference spectrum is recorded followed by recording of the sample spectrum. The spectrum of the sample is derived by ratioing the sample spectrum to the reference spectrum in providing a transmittance spectrum of the sample with all instrument response characteristics removed.
The single beam approach has several intrinsic disadvantages. For example, the spectrometer's optical alignment may drift during the interval between the sample and reference measurements. Also, the atmosphere within the instrument may change between the sample and reference measurements. This frequently results in a water and carbon dioxide spectrum being superimposed upon the sample spectrum. Because water and carbon dioxide are strong infrared absorbers, this is a significant and frequent problem in infrared (IR) spectroscopic measurements. It is therefore desirable to minimize the time between the sample and reference measurements or to eliminate this time interval, if possible.
Sensor and other electronics limitations are another intrinsic disadvantage of the single beam approach. For example, in FTIR measurements, the amplitude of the reference signal is very large while the actual sample spectrum is typically provided in a very weak component of the strong background signal. This is an especially serious problem in FTIR because the interferogram of the reference spectrum which is recorded is a very large signal. The typical dynamic range of a sample spectrum signal within the interferogram is one part in a million, or smaller. This dynamic range generally exceeds the signal-to-noise ratio sensitivity of the FTIR measuring equipment. It is also difficult to maintain detectors as well as interferometer electronics linear over the wide dynamic range required to accurately measure the sample and reference interferograms. System nonlinearities limit the capability of such single beam spectrometers to provide highly accurate quantitative measurements. Finally, the sensitivity of many FTIR measurements is limited by the dynamic range of the detector preamplifier as well as the dynamic range of the analog-to-digital converter. Failure to remove the large background signal from the small digital signal results in amplification and other processing of both signals so as to reduce the accuracy of the sample spectrum measurement.
The dynamic range problem is far more critical in the FTIR measurement than in the dispersive measurement. The interferogram, i.e., the Fourier transform of the spectrum, is the measured signal in the Fourier transform spectrometer. The interferogram has a much larger dynamic range than the spectrum. Typical measurements might require detecting a 1% sample spectrum, i.e., one part in one hundred. However, the sample feature in the interferogram is frequently on the order of one part in one hundred thousand.
Current dispersive spectrometer designs generally make use of a dual beam arrangement employing a "chopper" to sample first the sample beam and then the reference beam. The chopper alternates between the beams at about 30 to 60 times per second so as to place a small alternating sample signal on the large background reference signal. By locking in on the AC component of the detector output so as to filter out the large DC background component, the sample spectrum may be measured more accurately. This technique is in widespread use in spectrometer designs and is generally referred to as the "dual beam" spectrometer in contrast to the "single beam" spectrometer.
Fourier transform, or interferometric spectroscopy, in general has not been amenable to the "chopper" dual beam design. Most of the problems arise from the interferometric principle of measuring all frequencies simultaneously. Each optical frequency has its own modulated audio frequency component in the interferogram. If a chopper is used, a doubly modulated signal is generated. These two modulations must be at distinct frequencies in order to be separated. This generally requires the chopper frequency to be on the order of 50 to 100 kHz, which is too fast to generate mechanically. The double modulation approach has been applied to polarization measurements, where the polarization can be alternated at these high frequencies. Because of these design issues, FTIR has remained a "single beam" technique with all its inherent problems. FTIR spectrometers have been designed with separate sample and reference beams and frequently employ switching mirrors to allow the system to alternate in successive scans between the two beams. The switching typically requires a few seconds and the scan times are limited to a few seconds, with several scans in each beam taken alternately. This approach removes some of the long term spectrometer drift effects, but does not address the dynamic range problems discussed above. Although such FTIR systems have been marketed as "dual beam" spectrometers, they are not in fact dual beam systems.
It is inherent in interferometer designs that two beams with the interferogram modulation signal are generated. One beam is generally available for the sample measurement and one beam returns to the IR source and is thus not available for spectroscopic use. If flat mirrors are employed in the interferometer, the beam returning to the source is spatially superimposed on the incoming source beam. The interferogram signal on the beam that returns to the source is an exact complement of the interferogram signal that emanates from the other interferometer port such that when one signal goes positive, the other goes negative by the same amount. This effect can be deduced from the conservation of energy principle applied at the interferometer's beamsplitter. Because the interferometer neither generates nor absorbs energy, allowing one of these beams to pass through a reference and the other to pass through a sample, a difference signal could be measured directly because the two interferograms would exactly cancel except for the small difference presented by the sample. Efforts have been made in the past to harness both the beamsplitter transmitted and reflected beams to provide a dual beam capability in the spectrometer. Such efforts have met with only limited success.
One prior approach at a dual beam FTIR spectrometer involves the slight misalignment of a "flat mirror" interferometer to spatially separate the incoming radiation beam from the interferometer return beam. Because this misalignment cannot be stabilized, the two interferograms do not cancel in a reproducible manner. The availability of cube corner retroflectors in the early 1980s of sufficient accuracy for use in FTIR interferometers gave rise to other attempts at developing a dual beam spectrometer. A cube corner interferometer allows the incoming radiation beam to be spatially separated from the beam that returns to the source. Thus, rays that enter the bottom of a cube corner interferometer leave from the top and vice versa. With the incoming radiation entering the lower portion of the interferometer, the two radiation beams emanating from the upper half of the interferometer may be used for spectroscopic measurements. This arrangement provides two interferograms which are spatially separated, one that can be passed through a sample and the other through a reference path. The two inteferograms are 180.degree. out of phase and if properly combined will cancel the large background signal, leaving the small sample signal.
Various attempts have been made to construct such a dual beam spectrometer, but they have encountered a problem which limits their practicality. This problem arises from the inability to recombine two beams with sufficient precision. It has proven to be extremely difficult to bring the two separate beams back together in an optically precise manner. Not only have attempts to optically recombine the beams been unsuccessful, but attempts to electronically recombine electrical signals representing the beams in the detector and its associated electronics have also met with only limited success.
The present invention overcomes the aforementioned problems encountered in the prior art by providing a dual beam Fourier transform spectrometer wherein the source and detector locations in a conventional spectrometer are interchanged and which employs a Michelson interferometer with cube corner retroreflectors. Two beams generated from a single IR source are respectively directed through a sample and a reference and are subsequently combined into one beam containing a difference interferogram which is directed to a single detector.